Diseases caused by insufficient or abnormal blood vessel growth affect a wide range of tissues, and include heart and brain ischemia, hypertension, atherosclerosis, pulmonary fibrosis, diabetes, and osteoporosis. At the heart of angiogenesis lies the inflammatory response, orchestrated primarily by macrophages. Changes in the behavior of macrophages have major effects on angiogenesis and vascularization of tissue engineering scaffolds that are designed to replace damaged tissues. The proposed work has two major goals: 1) to define the relationship between macrophages and angiogenesis, and 2) to develop a novel biomaterial platform to control macrophage behavior in order to encourage vascularization by the body's own cells. In normal wound healing, macrophages first exhibit a pro-inflammatory M1 phenotype, and then switch to an alternatively activated M2 phenotype. Although angiogenesis is a part of normal wound healing, the relative contributions of M1 and M2 macrophages to angiogenesis are not well understood. M2 macrophages can also be subdivided into two different phenotypes, M2a and M2c, each with distinct but poorly understood effects on angiogenesis. The overarching hypothesis of this work is that M1 and M2 (both M2a and M2c) macrophages function synergistically and sequentially to promote the growth of new blood vessels. Co-culture experiments between polarized macrophages and endothelial cells will be used to investigate how their crosstalk affects both the M1-to-M2 transition of macrophages and the phenotypic changes in endothelial cells that lead to sprouting and stabilization of new blood vessels. A 3D model of angiogenesis that tracks the behavior of endothelial cells and pericytes over 5 days will be used to probe the effects of changing the timing, duration, and sequence of signals secreted by the different macrophage phenotypes. Finally, macrophage regulation of angiogenesis will be evaluated in vivo using novel immunomodulatory drug delivery systems designed to recapitulate the normal sequence of M1 and M2 macrophage activation. This project will investigate the mechanisms of macrophage regulation of successful blood vessel growth, which will enhance our understanding of how it goes awry in pathological conditions. In addition, this project will result in a novel and biomimetc approach to direct scaffold vascularization by controlling the actions of host macrophages.